Heretofore, there has been known an HUD device which is mounted on a movable object and displays a virtual image viewable by an occupant by projecting a display light onto a projection member through a projection window. The HUD device disclosed in Patent Literature 1 includes a display light projection unit that projects an image as a display light including multiple wavelengths in a visible region, and a light guide unit that guides display light from the display light projection unit to a projection window. The light guide unit has a cold mirror unit for reflection of a display light with the use of an optical multilayer film.
In this example, the cold mirror unit has a high reflectance in 450 to 480 nm, 530 to 560 nm, and 610 to 640 nm, and has a low reflectance in other wavelength regions of a visible light. In particular, there are wavelengths having two minimum reflectances around 510 nm and around 590 nm, and the two minimum reflectances are set at approximately 50% which are substantially equal to each other. An external light of sunlight or the like that enters the HUD device through a projection window from an external of HUD unit may be restricted from being reflected by the cold mirror unit and reaching a display light projection unit. In other words, a decrease in a lifetime due to a rise in a temperature of the display light projection unit may be reduced.
However, in the case of employing the cold mirror unit of Patent Literature 1, there is a concern that a display quality of a virtual image may be lowered due to a decrease in luminance and a change in chromaticity.